Cholecystokinin
|image=Control-of-stomach-acid-sec.png |caption=CCK identified at bottom right. |Symbol=CCK |AltSymbols= |HGNCid=1569 |Chromosome=3 |Arm= |Band= |LocusSupplementaryData=pter-p21 |ECnumber= |OMIM=118440 |EntrezGene=885 |RefSeq=NM_000729 |UniProt=P06307 |PDB= }} Cholecystokinin (CCK or CCK-PZ; from Greek chole, "bile"; cysto, "sac"; kinin, "move"; hence, move the bile-sac (gallbladder)) is a peptide hormone of the gastrointestinal system responsible for stimulating the digestion of fat and protein. Cholecystokinin, previously called pancreozymin, is synthesized by I-cells in the mucosal epithelium of the small intestine and secreted in the duodenum, the first segment of the small intestine, and causes the release of digestive enzymes and bile from the pancreas and gallbladder, respectively. It also acts as a hunger suppressant. Structure CCK is composed of varying numbers of amino acids depending on post-translational modification of the CCK gene product, preprocholecystokinin. Thus CCK is actually a family of hormones identified by number of amino acids, e.g., CCK58, CCK33, and CCK8. CCK58 assumes a helix-turn-helix configuration. Its existence was first suggested in 1905 by the British physiologist Joy Simcha Cohen. CCK is very similar in structure to gastrin, another of the gastrointestinal hormones. CCK and gastrin share the same five amino acids at their C-termini. Functions CCK mediates digestion in the small intestine by inhibiting gastric emptying and gastric acid secretion. It stimulates the acinar cells of the pancreas to release digestive enzymes and stimulates the secretion of a juice rich in pancreatic digestive enzymes, hence the old name pancreozymin. Together these enzymes catalyze the digestion of fat, protein, and carbohydrates. Thus, as the levels of the substances that stimulated the release of CCK drop, the concentration of the hormone drops as well. The release of CCK is also inhibited by somatostatin. Trypsin, a protease released by pancreatic acinar cells hydrolyzes CCK-releasing peptide and monitor peptide effectively turning off the additional signals to secrete CCK. Cholecystokinin in the body cannot cross the blood brain barrier, but certain parts of the hypothalamus and brainstem aren't protected by the barrier. Interactions Cholecystokinin has been shown to interact with the Cholecystokinin A receptor located mainly on pancreatic acinar cells and Cholecystokinin B receptor mostly in the brain and stomach. CCKB receptor also binds gastrin, a gastrointestinal hormone involved in stimulating gastric acid release and growth of the gastric mucosa. CCK has also been shown to interact with calcineurin in the pancreas. Calcineurin will go on to activate the transcription factors NFAT 1–3, which will stimulate hypertrophy and growth of the pancreas. CCK can be stimulated by a diet high in protein, or by protease inhibitors. Role in appetite and satiety CCK mediates a number of physiological processes, including digestion and satiety. It is released by I cells located in the mucosal epithelium of the small intestine (mostly in the duodenum and jejunum), neurons of the enteric nervous system and neurons in the brain. Release of CCK is stimulated by monitor peptide released by pancreatic acinar cells as well as CCK-releasing protein, a paracrine factor secreted by enterocytes in the gastrointestinal mucosa. In addition, release of acetylcholine by the parasympathetic nerve fibers of the vagus nerve also stimulate its secretion. The presence of fatty acids and/or certain amino acids in the chyme entering the duodenum is the greatest stimulator of CCK release. As a neuropeptide, CCK mediates satiety by acting on the CCK receptors distributed widely throughout the central nervous system. In humans, it has been suggested that CCK administration causes nausea and anxiety, and induces a satiating effect. CCK-4 is routinely used to induce anxiety in humans though certainly different forms of CCK are being shown to have highly variable effects. The mechanism for this hunger suppression is thought to be a decrease in the rate of gastric emptying. Cholecystokinin has been shown to interact with orexin neurons which control appetite and wakefulness (sleep). Cholecystokinin can have indirect effects on sleep regulation. The effects of CCK vary between individuals. For example, in rats, CCK administration significantly reduces hunger in young males, but is slightly less effective in older subjects, and even slightly less effective in females. The hunger-suppressive effects of CCK also are reduced in obese rats. Role in anxiety CCK also has stimulatory effects on the vagus nerve, effects that can be inhibited by capsaicin. The stimulatory effects of CCK oppose those of ghrelin, which has been shown to inhibit the vagus nerve. The CCK tetrapeptide fragment CCK-4 (Trp-Met-Asp-Phe-NH2) reliably causes anxiety when administered to humans, and is commonly used in scientific research to induce panic attacks for the purpose of testing new anxiolytic drugs. Role in drug tolerance Recent evidence has suggested that it also plays a major role in inducing drug tolerance to opioids like morphine and heroin, and is partly implicated in experiences of pain hypersensitivity during opioid withdrawal. See also *Antianalgesia *Cholecystokinin antagonist *Cholecystokinin receptor *Cholecystokinin A receptor *Cholecystokinin B receptor *Cholecystokinin tetrapeptide *Hormones *Neuropeptides *Neurotransmitters *Peptides *Proglumide References External links * Cholecystokinin, NIH/NLM Medical Subject Headings Category:Cholecystokinin Category:Hormones Category:Intestinal hormones Category:Peptides Category:Neuropeptides Category:Neurotransmitters Categroy:Satiety